Interpretive Summary: Switchgrass has been identified as important plant for biofuel and biomass production in the U.S. This grass is native to North American grasslands and tall grass prairies. Currently, extensive research is being done to improve yields and conversion properties for biofuels. However, little research has been done to improve the genetic resistance to pests and diseases. Rust has become a major disease on switchgrass, where large acreage is planted. Severe outbreaks of rust were observed in fields trials of a diverse collection of switchgrass accessions in Oklahoma during 2007 to 2010. Based on morphological characters, the rust pathogen was identified as Puccinia emaculata. Several different rust pathogens are known to infect switchgrass, and therefore a DNA based diagnostic assay was developed for rapid identification of P. emaculata. Four representative cultivars of switchgrass were screened for resistance to P. emaculata in the field and in controlled growth chambers. High levels of variation in rust resistance were observed between cultivars and between different genotypes within cultivars. These results demonstrated that significant improvement in rust resistance in switchgrass cultivars would be obtained through breeding programs that incorporate rust resistance screening. These results will be used by plant breeders and plant scientist working in the biofuel production.

Technical Abstract:
Several fungal pathogens have been identified on ornamental and native stands of switchgrass (Panicum virgatum L.). Diseases of switchgrass have been largely neglected and pathogens are likely to become the major limiting factor to biomass yield and quality, especially when planted in large acreage and monocultures. Rust is a major emerging disease in switchgrass. Rust spores were collected from switchgrass research fields in Oklahoma. Based on teliospore morphology and ITS-based diagnostic primers, the rust pathogen was identified as Puccinia emaculata. Furthermore, to identify genetically diverse source(s) of rust resistance, several switchgrass genotypes from both upland (cv. 'Summer' and 'Cave-in-Rock') and lowland (cv. 'Alamo' and 'Kanlow') ecotypes were evaluated in Ardmore, Oklahoma during 2008 and 2009 and in growth chamber assays. Field and growth chamber evaluations revealed a high degree of genetic variation within and among switchgrass cultivars. In general, Alamo and Kanlow showed moderate resistance to P. emaculata, while Summer was highly susceptible. Distinct ecotypic variations for reactions to rust were also prevalent with the lowlands maintained high level of resistance. These results suggest the potential for improvement of rust resistance via the selection of resistant individuals within the cultivars currently available. Further, the selection pressure on the pathogen would also be reduced by employing several different cultivars in production-scale situations.